The present invention relates to a method for influencing the stereo separation of an audio signal that is to be reproduced, and to a radio receiver for carrying out the method.
Modern radio transmitters emit radio signals whose signal format makes possible the transmission of stereophonic audio signals and allows the spatial sound of a stereophonic reproduction to be clearly audible at the receiver end.
This advantage is lost if a radio signal that is to be received is present at the receiving location with only a low field strength, since interference-free channel separation requires a minimum received field strength for a radio signal received at the receiving location. Radio receivers, in particular, car radios, have therefore been equipped for some time with a switching transition or continuous transition from stereophonic to monophonic reception, which depends on the received field strength of a received radio signal. In radio receivers of this kind, the channel separation is automatically reduced when there is a decrease in the received field strength.
Further reception interference events often occur in addition to the fluctuation in the received field strength of a received radio signal, for example, multipath reception of a tuned-in radio transmitter. Under such reception conditions it is disadvantageously noticeable, even with the aforesaid stereo radio receivers in which the continuous mono-stereo transition is set for optimum channel separation, that good stereo reception is suddenly disrupted when the vehicle enters a multipath reception area; this often occurs only over short distances. Stereo-capable radio receivers have therefore been equipped with a detector for interference events, for example, multipath reception of the tuned-in transmitter, whose output signal controls a switcher which forces the receiver to switch to monophonic reproduction of the received radio signal.
European Patent No. 0 617 519 describes a circuit arrangement for a radio receiver of this kind in which a signal indicating the received field strength is derived from the received field strength of a received radio signal; in which an interference signal indicating the reception interference events is derived from the received radio signal; and in which the signal indicating the received field strength, and the interference signal, are combined in multiplicative fashion to form a factor, delivered to a stereo decoder, for influencing the stereo channel separation of an audio signal that is to be reproduced.
On journeys affected by short-term multipath reception, e.g., on streets with tall buildings or in mountainous areas, alternations between single-path reception and multipath reception often occur very quickly, and result in correspondingly frequent switchovers from stereo reproduction to monophonic reproduction of a stereo transmission. This effect has long been perceived as so annoying that drivers in mountainous areas switch off the stereo decoder when receiving modern stereo transmitters, and dispense entirely with stereo reproduction.
Conventional radio receivers have the capability for analyzing data transmitted by way of the Radio Data System (RDS), which for the purpose of optimizing the reception quality of a received radio program are tuned briefly from a currently tuned-in transmission frequency to an alternative transmission frequency over which the same program is being transmitted; during the dwell time on the alternative transmission frequency, its reception quality is determined, and then the receiver is set to the transmission frequency having the best reception conditions. The information concerning the alternative transmission frequency is derived, for example, from the alternative frequency (AF) data transmitted over the Radio Data System.
Tuning the radio receiver to an alternative transmission frequency, whose received field strength may differ greatly from that of the transmission frequency originally tuned in, causes sudden and thus annoying changes in the stereo channel separation of a reproduced audio signal.
A method and a radio receiver according to the present invention have an advantage in that if reception conditions fluctuate greatly or if reception interference events (for example, multipath reception) occur frequently, rapid and frequent alternation between stereophonic and monophonic reproduction, and the negative auditory impression associated therewith, can be suppressed in favor of partially or completely monophonic reproduction.
It is particularly advantageous to embody the characteristic curves for evaluating the field strength signals indicating the received field strength of a radio signal in the form of step functions, since for the situation in which the method or the radio receiver according to the present invention is implemented with digital technology, such functions are particularly easy to implement.
Specifically, in the case of digital signal processing, it is also advantageous to implement a signal indicating the frequency of reception interference events by way of a counter which is incremented when an interference event occurs and decremented when interference is absent, since the clock signal required for this counter can easily be derived, for example, from the system clock, e.g., from the bit clock of the digital signals involved.
Lastly, it is particularly advantageous, in the case of radio receivers for the reception of Radio Data signals which perform automatic alternative frequency tests to optimize reception quality, to hold the channel separation auxiliary signal at its most recent value for the duration of one alternative frequency test, and to change it, if applicable, only after the alternative frequency test; in this fashion, large fluctuations in stereo channel separation during alternative frequency tests can be prevented.